Mars: Just Scratching the Surface?

Mystery of Spiral Patterned Poles Unique in Solar System

The spiral troughs of Mars’ polar ice caps have been called the most enigmatic landforms in the solar system. The deep canyons spiraling out from the Red Planet’s North and South poles cover hundreds of miles. No other planet has such structures.

A new model of trough formation suggests that heating and cooling alone are sufficient to form the unusual patterns. Previous explanations had focuse on alternate melting and refreezing cycles but also required wind or shifting ice caps.

"I applied specific parameters that were appropriate to Mars and out of that came spirals that were not just spirals, but spirals that had exactly the shape we see on Mars." said Jon Pelletier, an assistant professor of geosciences at the University of Arizona in Tucson. "They had the right spacing, they had the right curvature, they had the right relationship to one another."

His report, "How do spiral troughs form on Mars?," is published in the April issue of the journal Geology. One of his computer simulations of the troughs graces the cover.

How the icy canyons formed in a spiral has puzzled scientists since the pattern was first spotted by the Viking spacecraft in 1976.

Pelletier, a geomorphologist who studies landforms on Earth such as sand dunes and river channels, has a fondness for natural patterns that are regularly spaced.

Spirals fit the bill, and while perusing a book on mathematical patterns in biology, he was struck by the spiral shape formed by slime molds. He wondered whether the mathematical equation that described how the slime mold grew could also be applied to geological processes.

"There’s a recipe for getting spirals to form," he said. So he tried it out, using information that described the situation on Mars.

Temperatures on Mars are below freezing most of the year. During very brief periods during the summer, temperatures on the polar ice caps get just high enough to let the ice melt a bit, Pelletier said.

He proposes that during that time, cracks or nicks in the ice’s surface that present a steep side toward the sun might melt a bit, deepening and widening the crack. Heat from the sun also diffuses through the ice.

Much as ice cubes evaporate inside a freezer, on Mars, the melting ice vaporizes rather than becoming liquid water.

The water vapor, when it hits the cold, shady side of the little canyon, condenses and refreezes. So the canyon expands and deepens because one side is heated occasionally while the other side always remains cold.

"The ambient temperatures on Mars are just right to create this form. And that’s not true anywhere else in the solar system," he said. "The spirals are created because melting is focused in a particular place."

Pelletier said the differential melting and refreezing is the key to the formation of Mars’ spiral troughs.

So he put mathematical descriptions of the heating and cooling cycles into the spiral-generating equation and ran computer simulations to predict what would occur over thousands of such cycles. He did not include wind or movement of polar ice caps in his model.

The computer made patterns that match what’s seen on Mars, even down to the imperfections in the spirals.

"The model I have predicts the spacing between these things, how they’re curved, and how they evolve over time to create spiral feature," he said.

"A lot of planetary sciences is about making educated guesses about the imagery that we see. We can’t go there, we can’t do do field experiments," he said. "The development of numerical models provides strong suggestions as to what’s essential to create the form that we see," and allows scientists to test their assumptions, he said.

University of Buffalo volcanologist, Tracy Gregg, shares an interest in the martian poles also because of their unusual heating and cooling patterns: "Higher latitudes are interesting to me because that’s where the large volcanoes are, and there’s more opportunities for magma/water interactions. Those interactions are probably extremely important for the origin and evolution of life. "

What’s Next

As a pioneer on the team that got the first close-up orbital images on Mariner 9 and a current member of the imaging team for NASA’s Mars Global Surveyor mission, Arizona’s Bill Hartmann told Astrobiology Magazine, some of the most fascinating and unexplored areas of Mars are polar.

"Definitely the underground ice is closer to the surface," said Hartmann, "the further you go from the equator toward the poles. One piece of evidence is the mud-like ejecta from shallow craters at high latitudes, and another piece of the puzzle is patterned ground, as observed in tundra regions of Earth. Icy soils give at least the potential for moist soils and life-nurturing environments. If you land near the pole in summer, you may sit on ice or have the ice within a foot or two of the surface, as shown by Mars Odyssey findings. And remember, such a site can have 24 hours/day of sunlight in summer, a bit weaker than at equator, but available all day long to charge the [solar] batteries. (By coincidence, Mars has about a 24 hour day like Earth). As a result of these ideas the Phoenix mission, now being built, will land near the pole and scratch the ground to look for signs of life in the ice-rich soils."